The present invention generally relates to calibrating an oscillator circuit in a transmitter. More particularly, the present invention relates to a method and a system for automatically calibrating an oscillator circuit that shifts in frequency, primarily due to aging of an oscillator crystal and/or seasonal changes in ambient temperature.
Most radio frequency (RF) communications equipment require an accurate and precise reference frequency with low frequency drift characteristics to maintain stable communications with other RF communication equipment. In some applications, the FCC imposes the requirement that an RF transmitter maintain very high frequency accuracy over the life span of the equipment and that the transmitters operate within narrow channels. In outdoor applications, the frequency accuracy must be maintained over a wide temperature range. The accuracy of the transmit frequency is directly related to the accuracy of the oscillator circuit used in the transmitter.
Crystal oscillators are often used in RF transmission equipment to provide the requisite reference frequencies. However, crystal oscillators are susceptible to frequency drift, primarily due to crystal aging, and to frequency shifts that are primarily caused by variations in ambient temperature. One approach to resolving issues of frequency drift caused by crystal aging and ambient temperature variations involves using voltage-controlled, temperature-compensated crystal oscillators. A temperature-compensated crystal oscillator is typically an open-loop device that includes a temperature sensing circuit that outputs a control signal to a frequency tuning circuit connected to the crystal oscillator. Unfortunately, temperature-compensated crystal oscillators often include additional automatic frequency control (AFC) loops and analog tuning devices that occupy valuable circuit board space and consume power. In addition, temperature-compensated crystal oscillators circuits typically have a limited ability to compensate for oscillator frequency drift due to crystal oscillator aging. Examples of these types of crystal oscillator circuits include U.S. Pat. Nos. 6,064,270, 5,572,169, and 5,552,749.
Another approach to minimizing aging effects of the crystal oscillator includes enclosing the oscillator inside a miniature oven to reduce the potential for stresses on the crystal caused by ambient temperature changes. Unfortunately, oven controlled oscillators are expensive and consume large amounts of power. Consequently, oven controlled crystal oscillators are impractical for applications requiring low cost, low power consuming temperature-compensating oscillator circuits.
Crystal oscillators that are exposed to varying ambient temperatures over a long period of time exhibit changes in the crystal structure that cause the crystal""s resonant frequency to shift. By applying a voltage to a calibration pin on the oscillator, the frequency can be adjusted back to the original specified frequency. A typical error due to aging is about 0.5 parts per million (PPM) drift in frequency per year. In some applications, the FCC usually requires that the frequency drift due to aging not exceed about 1.5 PPM over the life of the transmitter. Temperature-compensation crystal oscillator circuits typically are designed to maintain the oscillator within 1.0 PPM, in which case the oscillator circuit must be manually recalibrated at least once a year in order to ensure compliance with FCC requirements. For oscillators used in transmitters that have restricted access, such as utility pole mounted transmitters, calibrating these oscillators on an annual basis is simply impractical.
Hence, a need exists for a method and a system for compensating the reference frequency of transmitters with crystal oscillator circuits that is low cost, consumes low power and substantially reduces any labor cost involved in calibrating the oscillator circuit.
Various embodiments of the present invention are directed to addressing the above and other needs in connection with compensating for reference frequency drift in a communications device having a crystal oscillator circuit that uses a network time protocol (NTP) as a time reference for calibrating the reference frequency without the need for manual recalibration.
According to one aspect of the invention, a method of compensating for reference frequency drift in a communications device utilizes time stamps obtained from a networked reference clock arrangement to adjust a local crystal oscillator circuit of the communications device. The local oscillator circuit generates a reference frequency signal and operates synchronously with a clock circuit of a microprocessor arrangement. The reference frequency compensation method includes generating a clock circuit time reference for the clock circuit by dividing the reference frequency signal by a divisor and obtaining a synchronization time stamp from a networked reference clock arrangement that is operably connected to the microprocessor arrangement. The clock circuit is then synchronized to the synchronization time stamp. The method also includes obtaining a calibration time stamp from the networked reference clock arrangement after a predetermined time duration has transpired from synchronization of the clock circuit and comparing the calibration time stamp to a current time of the clock circuit. The method further includes synchronizing the current time of the clock circuit with the calibration time stamp and adjusting the local oscillator circuit as a function of a time variation between the calibration time stamp and the current time of the clock circuit.
According to another aspect of the invention, a reference frequency compensation circuit arrangement having a local crystal oscillator circuit utilizes time stamps from a networked reference clock to calibrate the local crystal oscillator. The frequency compensation circuit arrangement includes a dividing circuit that divides a reference frequency signal from the local oscillator circuit and generates a clock circuit time reference. The compensation circuit arrangement also includes a microprocessor arrangement with a clock circuit that is operably connected to a networked reference clock arrangement, the microprocessor arrangement can obtain a synchronization time stamp from the reference clock arrangement and can synchronize the clock circuit time reference to the synchronization time stamp. The microprocessor arrangement can also obtain a calibration time stamp from the networked reference clock arrangement after a predetermined time duration from the synchronization of the clock circuit has transpired. The microprocessor then compares the calibration time stamp to a current time stamp of the clock circuit. The microprocessor arrangement then synchronizes the current time reference of the clock circuit with the calibration time stamp and adjusts the local oscillator circuit as a function of a time variation between the calibration time stamp and the current time of the clock circuit. In a related embodiment, the time stamps are obtained from a fixed clock reference, such as an atomic clock. In the example, the oscillator can also be compensated for seasonal changes in ambient temperature.
The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures in the detailed description that follow more particularly exemplify these embodiments.